1. Field of the Invention
The present invention relates to an R—Fe—B rare-earth magnet and a method of producing the same.
2. Description of Related Art
In the prior art, neodymium (Nd) and/or praseodymium (Pr) have primarily been used as the rare-earth element R of an R—Fe—B rare-earth magnet because the use of these rare-earth elements provides particularly desirable magnetic properties.
In recent years, the variety of applications of R—Fe—B magnets has expanded, and the Nd and Pr consumption is increasing rapidly. Accordingly, there is a strong demand to improve the efficiency of use of Nd and Pr, which are precious natural resources, and for reducing the material cost of an R—Fe—B magnet.
The simplest way to reduce the Nd and Pr consumption is to substitute Nd and Pr with another rare-earth element that functions similarly to Nd and Pr. It is known in the art, however, that the magnetic properties, such as magnetization, deteriorate when a rare-earth element other than Nd and Pr is added to an R—Fe—B rare-earth magnet. Therefore, rare-earth elements other than Nd and Pr have rarely been used in R—Fe—B rare-earth magnets.
For example, when an R—Fe—B alloy is made by melting and solidifying a material alloy with Yttrium (Y), a rare-earth element, being added to the material along with Nd, Y is taken into the main phase of the alloy. The main phase of an R—Fe—B alloy principally has a tetragonal R2Fe14B type crystalline structure. It is known in the art that the highest magnetization is exhibited when R is Nd and/or Pr (and dysprosium (Dy), terbium (Tb), etc., substituting part of Nd and/or Pr). When R in the R2Fe14B crystalline structure forming the main phase is substituted either partially or entirely with a rare-earth element such as Y, the magnetization substantially decreases.
An R—Fe—B magnet with cerium (Ce), a rare-earth element like Nd and Pr, added thereto is disclosed in the report of Proc. 16th Inter. Workshop on Rare Earth Magnets and their Applications, 2000. P99. According to the report, the residual magnetic flux density or remanence Br decreases linearly due to the addition of Ce.
In view of the above, it is believed that the addition of any magnetization-decreasing rare-earth element R, other than Nd, Pr, Dy, and Tb, should be avoided as much as possible.
Nd and/or Pr not only form a main phase but also exist in a grain boundary phase, and play an important roll of forming a liquid phase in a sintering process. However, Nd and/or Pr existing in a grain boundary phase form a non-magnetic phase and do not contribute to the improvement of magnetization. In other words, a part of Nd and/or Pr is always consumed for the formation of a non-magnetic phase, failing to directly contribute to the magnetic properties.
In order to efficiently use Nd and/or Pr so as to effectively achieve desirable magnetic properties, it is preferred that most of Nd and/or Pr is taken into the R2Fe14B crystal phase. However, techniques for realizing this did not exist in the prior art.
In the prior art, a part of Fe in the main phase having a tetragonal R2Fe14B crystalline structure is substituted with cobalt (Co) by adding Co to a material alloy in order to improve the heat resistance of an R—Fe—B rare-earth magnet. When a part of Fe is substituted with Co, the Curie temperature of the main phase increases, whereby desirable magnetic properties can be exhibited even at higher temperatures.
In recent years, in some fields of art such as motors for use in automobiles, there is a demand for a magnet having a higher performance and hence a demand for the use of an R—Fe—B rare-earth magnet having a higher performance than that of a ferrite magnet. However, the heat resistance of an R—Fe—B rare-earth magnet is not sufficient for use under a high temperature environment such as those experienced by a motor in an automobile. Accordingly, there is a strong demand for further improving the heat resistance of R—Fe—B rare-earth magnets.
It is believed that in order to further improve the heat resistance of an R—Fe—B rare-earth magnet, it is preferable to add more Co. However, Co added to a material alloy not only substitutes Fe in the main phase of a sintered magnet but also exists in a grain boundary phase to form an NdCo2 compound and/or a PrCO2 compound therein. Thus, a part of Co added is not used for substituting Fe but is wasted in the grain boundary phase. Another problem is that the above compounds are a ferromagnetic substance and thus decreases the coercive force of the sintered magnet. Therefore, simply increasing the amount of Co to be added is not an effective way to substitute Fe in the main phase, and doing do can substantially decrease the coercive force of an R—Fe—B rare-earth magnet.